Recently, with the increase in the number of WDM signals in wavelength-division multiplexing or in the modulating speed of an optical signal, transmission capacity, which is the amounts of information transmissible over one optical fiber, has also increased. However, it is considered that the transmission capacity reaches the limit at approximately 10 T(tera) bits/s. The reason is that a wavelength band usable to the optical transmission reaches a limit value defined by a wavelength band of an optical fiber amplifier (when summing C, L, and S bands, about 80 nm corresponds to 10 THz) and thus, there is no room to increase the number of signal wavelengths.
In order to further increase the transmission capacity of the optical fiber, there is a need to study a modulation format and increase efficiency of frequency usage. In the field of wireless radio communication, a multilevel modulation technology has been widely used since the 1960's, such that high-efficiency transmission, such high efficiency of frequency usage exceeding 10, can be achieved. The multilevel modulation is promising even in the optical signal transmission using the optical fiber and therefore, numerous studies thereon have been made up to now.
For example, a Quadrature Phase Shift Keying (QPSK) system that performs quaternary (4-level) phase modulation is proposed in “10 Gbits/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration” OFC2002, paper PD-FD6, 2003 (Non-Patent Document 1). Further, the inventors propose 16-level amplitude-and-phase modulation combining quaternary amplitude modulation and quaternary phase modulation in “Proposal and Demonstration of 10-Gsymbol/sec 16-ary (40 Gbits/s) Optical Modulation/Demodulation Scheme”, paper We3.4.5, ECOC 2004, 2004 (Non-Patent Document 2).
Various optical multilevel modulation have been studied as above, as a factor that disturbs the practical use of the multilevel modulation, there is an inter-symbol-interference between transmitting waveforms. Further, since a signal bit rate used for the optical fiber communication reaches 10 Gbits/s to 40 Gbits/s, a very high-speed multilevel optical modulation technology is required. The speed of the multilevel optical modulated signal is 100 to 1000 times a typical multilevel radio signal, and in a process for generating the multilevel optical modulated signals, the signal waveforms are remarkably degraded due to frequency responses of the optical modulator and RF portions and additive operation of a group of RF signals.
In order to help understand the present invention to be described later, the deterioration of the signal waveforms and the inter-symbol-interference caused in the process of generating the multilevel optical signals will be described with reference to FIGS. 4A to 4C.
FIG. 4(A) shows an eye-pattern of a waveform of a binary electrical signal of 10 Gbits/s to be used in generating multilevel signals. Even in the waveform of the simple digital signal such as a binary signal, when the signal bit rate is 10 Gbits/s or higher, it becomes difficult to obtain an ideal rectangular waveform. Since two levels above and below the waveform of the digital signal, that is, marks and spaces are degraded by the inter-symbol-interference (ISI), the waveform of the digital signal generally has certain width in an upper and lower direction as shown in FIG. 4A. As a result, it is known that a height of an eye-opening located at a central part of the waveform of the digital signal is reduced, leading to deterioration of receiver sensitivity and chromatic dispersion tolerance.
If the signal bit rate exceeds 10 Gbits/s, it becomes difficult to directly generate the multilevel signal. Accordingly, such a high-speed multilevel signal is generally generated by adding binary electrical digital signals having different amplitude values. For example, in the case of generating a high-speed quaternary digital signal, two sequences of binary digital signals having the same bit rate are generated independently and added at an amplitude ratio of 2:1 in a state where their bit timings are matched with each other.
For example, if a binary electrical signal having amplitude “2” shown in FIG. 4(A) and a binary electrical signal having amplitude “1” shown in FIG. 4(B) are added by using an RF signal divider, a quaternary electrical signal having four voltage levels arranged at regular intervals is generated, as shown in FIG. 4(C). In this case, when two sequences of RF signals are added, the inter-symbol-interference increases due to signal reflection caused in circuit components or undesired frequency response of circuit components, etc. As a result, the waveform of the multilevel signal (in this example, quaternary signal) is significantly degraded as shown in FIG. 4C.
On the other hand, in order to generate multilevel amplitude modulated light, it is necessary to perform the optical amplitude modulation by applying the above-mentioned multilevel electrical signal to the high-speed optical modulator. In general, voltage amplitude required for driving the optical modulator is 2 to 5 V; it is an extremely large amplitude value for the RF signal. Therefore, voltage amplification using a driver amplifier is required in driving the optical modulator. However, the multilevel waveform is largely degraded due to nonlinearity of the amplifier characteristics, limitations of output saturation, and frequency bandwidth, and peaking, etc.
A Mach-Zehnder (MZ) optical modulator having been widely used for the optical modulation can advantageously control the chirp of an optical phase involved in the optical modulation with high accuracy and realize good modulation characteristics (optical transmittance of driving voltage) in a wideband. However, the modulation characteristic of the MZ type optical modulator exhibits a sinusoidal wave with respect to the applied voltage as shown in a graph of FIG. 5.
In the simple binary amplitude modulation that is widely used for the high-speed optical fiber communication, it is normal to match the levels of the mark and the space of the electrical signal with a bottom and a peak of the modulation characteristic of the sinusoidal wave shown in FIG. 5. In this case, even when the voltage values of the mark level and the space level are fluctuated to some degree due to the inter-symbol-interference, it possible to obtain an extremely excellent output waveform because the fluctuation in the intensity of the optical signal is suppressed. This is because an inclination of the modulation characteristic with respect to the applied voltage becomes approximately 0 at both points of the bottom and the peak, which is known as a waveform rectifying effect of the MZ type optical modulator.
When the MZ type optical modulator is used for the quaternary optical amplitude modulation, as shown in FIG. 5, the quaternary electrical signal is made match the shoulder portions of the modulation/extinction characteristics of the sinusoidal wave so that different optical power is obtained in corresponding to four voltage levels of the quaternary electrical signal. At this time, the waveform rectifying effect of the above-mentioned MZ type optical modulator can be obtained by matching two voltage levels at both ends of the quaternary electrical signal with the peak and the bottom of the extinction characteristic, respectively. However, there is a problem that the inter-symbol-interference cannot be suppressed because the waveform rectifying effect is not achieved in two voltage levels at a center of the quaternary electrical signal.
In the high-efficiency optical multilevel signal transmission, a multilevel amplitude-and-phase modulation, which applies the phase modulation simultaneously with the amplitude modulation to the optical signal, has been studied as described in the Non-Patent document 2. In the optical amplitude modulation used herein, in order to utilize the optical phase information at any time, it is necessary to intentionally set the extinction ratio of the optical signal to be lower so that the optical level L0 having the minimum amplitude becomes larger than 0. In this case, as shown in FIG. 5, even for the optical level L0 having the weakest power, the waveform rectifying effect cannot be obtained and the large waveform deterioration occurs in the quaternary waveform of the output light. As such, the occurrence of the inter-symbol-interference becomes a big problem in the generation of the optical amplitude modulation.
On the other hand, in the field of the optical phase modulation, a modulator that suppresses the inter-symbol-interference is proposed. The most basic configuration is a binary optical phase modulator with a modulation factor π that uses a single drive chirp-less MZ type optical modulator as shown in FIG. 6.
In the single drive chirp-less MZ type optical modulator 120 shown in FIG. 6, input light 101 of a continuous wave (CW), which is input from an input optical path 102, propagates through a waveguide structure in the modulator and is divided into two optical waveguides 127-1 and 127-2 by an optical splitter 103. Two lights passing through these optical waveguides are combined by an optical coupler 109 and output to an output optical path 110 as output light 126 having been subjected to the binary phase modulation of a modulation factor π.
A traveling-wave electrode 121 for modulation signal is disposed between two waveguides 127-1 and 127-2 on a base plate of the modulator. A modulation signal input terminal 122 is connected to one terminal of the traveling-wave electrode 121 and a terminating resistance 125 is connected to the other terminal thereof. A high-speed binary electrical digital signal applied to the modulation signal input terminal 122 is absorbed by the terminating resistance 125 after propagating along the electrode 121. During the binary electrical digital signal is propagating along the traveling-wave electrode 121, the binary electrical digital signal generates an electric field which causes an electro-optic effect for two waveguides 127-1 and 127-2. As a result, phase difference occurs between two waveguides and the transmission strength of the optical signal changes in the sinusoidal wave manner according to signal voltage V applied to the traveling-wave electrode 121 as shown in FIG. 7.
In the above-mentioned single drive MZ type optical modulator, it is known that the phase chirp involved in the power change of the optical waveform can be disregarded (chirp-less) because the phase difference occurring in two waveguides 127-1 and 127-2 is an anti-phase to each other, and that the phase of the output optical signal is temporarily switched by π at a point where the optical signal power is 0 as shown in FIG. 7. Further, the phase of the above-mentioned sinusoidal wave can be shifted into any voltage position by a bias voltage applied to a biasing electrode 123 via a terminal 124.
When the above-mentioned single drive chirp-less MZ modulator is used in the binary optical phase modulation, as shown in FIG. 7, the average level L0 of the space and the average level L1 of the mark of the binary electrical digital signal are made match with the two peaks of the sinusoidal wave optical transmission characteristic, by matching the amplitude of the binary electrical digital signal applied to the traveling-wave electrode 121 with the sinusoidal wave period (2Vπ) of the optical transmission characteristic and by adjusting the bias voltage. In this case, the phase of the optical signal is switched to 0 when the binary electrical digital signal is space and to π when the binary electrical digital signal is the mark. Since both voltages of the mark and space are matched with the peak portion of the MZ type modulator transmission characteristic, the waveform rectifying effect is obtained with respect to the fluctuation of the voltage waveform and it possible to suppress the fluctuation of the optical power.
FIG. 8(A) shows the optical power waveform obtained through the experiment in the binary optical phase modulator using the single drive chirp-less MZ type optical modulator. It can be appreciated from the waveform that the fluctuation of the optical power occurs only in boundary portions of each bit, the power variation is suppressed in the vicinity of the center of the waveform, and the optical power is constant.
FIG. 8(B) shows signal constellation of the optical signal in the above-mentioned binary optical phase modulator. In this constellation, the complex optical field at a central time of each optical signal bit is plotted on a complex plane. The distance (radius) from the origin indicates the field amplitude and an angle from the I-axis indicates the phase of the optical field. As the amplitude on the I-axis is constant and signal points are located at two points of a phase 0 and a phase π, this binary optical phase modulator can generate approximately ideal phase modulated light having little extension in a radial direction and an angular direction and almost no inter-symbol-interference.
Further, it is known that above-mentioned binary optical phase modulation of the modulation factor π can be realized, almost in the same manner, by a dual drive MZ type modulator having individual modulation electrodes in two optical waveguides in the modulator. In this case, the modulating operation may be performed at the same operation points by applying binary digital signals having the same amplitude (Vπ) and anti-phase to each other to two modulation electrodes at the same timing.
FIG. 9 shows a four-level quadrature optical phase modulator 133 in the prior art that is configured using the above-mentioned binary phase modulator. As this kind of quadrature phase modulator, for example, a modulator using a semiconductor MZ type optical modulator is disclosed in Non-Patent Document 1.
Input light 101 of continuous wave input from the input optical path 102 is split into two optical paths 104-1 and 104-2 by the optical splitter 103. The optical paths 104-1 and 104-2 are formed by optical waveguides. Above-mentioned single drive MZ type binary optical phase modulators 105-1 and 105-2 are located on both the optical paths, and binary electrical digital signals having the same bit rate are independently supplied to modulation signal input terminals 112-1 and 112-2. These binary electrical digital signals are adjusted to have signal delay amounts so that the bit timings are matched with each other. By applying the above-mentioned binary electric digital signals, binary phase modulation of modulation factor π is applied to the light passing through the optical paths 104-1 and 104-2, respectively.
In one optical path 104-1, a π/2 optical phase shifter 131 is inserted at a rear of the binary optical phase modulator 105-1 so that the phase of the binary phase modulated light is rotated by π/2. As a result, the signal constellation of the binary phase modulated light 107-1 passing through the optical path 104-1 becomes in a state as shown in FIG. 10(A), and the signal constellation of the binary phase modulated light 107-2 passing through the other optical path 104-2 becomes in a state as shown in FIG. 8(B).
These binary phase modulated lights are input to an optical coupler 109 in the same optical power and interfere with each other. Since the phase of the binary phase modulated light is in the state of any of two signal points shown in FIG. 10(A) or FIG. 8(B) momentary, the signal constellation of an output light 134 from the optical coupler 109 includes, as shown in FIG. 10(B), signal points obtained by performing vector synthesis on signal points or phase points of FIG. 10 (A) and signal points of FIG. 8 (B). The signal points of the output light 134 are four points of π/4, 3 π/4, −3 π/4, and −π/4, which are deviated by 90° to each other, and the output light 134 is quadrature quaternary phase modulated light.
Since the quadrature quaternary phase modulator 133 performs vector synthesis on the two binary phase modulated lights having no signal point extension (inter-symbol-interference), as far as the synthesis angle is stable, the output light 134 becomes ideal quaternary phase modulated light having no inter-symbol-interference.
FIG. 11 shows the configuration of binary optical phase modulator 130 having an arbitrary modulation factor shown in Japanese Patent Publication No. 2004-348112 (U.S. Pat. No. 6,798,557: Patent Document 1).
Although the binary optical phase modulator 130 shown here has approximately the same structure as the quadrature quaternary phase modulator 133 shown in FIG. 9, there is no optical modulator in the optical path 104-2 having been provided with a π/2 optical phase shifter 131. Therefore, the binary phase modulated light 107 and the continuous wave light (CW light) 108 are combined by the optical coupler 109. The binary phase modulated light 107 and the continuous wave light 108 are not the same in terms of power and their amplitude ratio is intentionally set 1:a.
FIG. 12(A) shows the signal constellation of the continuous wave light 108 having amplitude of “1” and FIG. 12(B) shows the signal constellation of the binary phase modulated light 107. As a result of π/2 phase shift, the continuous wave light 108 has an apex at points (0, 1) on a Q-axis. The signal point of the binary phase modulated light 107 is any one of (−a, 0) and (a, 0). Therefore, the signal point of the output light 132 obtained by interfering the two optical signals by the optical coupler 109 is any one of (−a, 1) and (a, 1) as shown in FIG. 12(C). It can be appreciated that because these two points have the same distance from the origin point to each other and the phase angles thereof are changed by ±φ=arctan(a), it becomes phase modulated light having modulation amount 2φ.
The binary optical phase modulator 130 can optionally set the modulation phase angle φ by adjusting the ratio “a” of the two optical signals 107 and 108 and can suppress the occurrence of inter-symbol-interference in both the phase direction and the amplitude direction, likewise the above-mentioned quadrature modulator. Patent Document 1 discloses that multilevel phase modulators such as 8-level, 16-level and the like can be configured by connecting phase modulators having phase modulation amount optionally set to π, π/2, and π/4, in a multi-stage tandem connection having a binary branch construction.
Non-Patent Document 1:
R. A. Griffin, et. al., “10 Gbits/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration” OFC2002, paper PD-FD6, 2003
Non-Patent Document 2:
Kenro Sekine, Nobuhiko Kikuchi, Shinya Sasaki, Shigenori Hayase and Chie Hasegawa, “Proposal and Demonstration of 10-Gsymbol/sec 16-ary (40 Gbits/s) Optical Modulation/Demodulation Scheme”, paper We3.4.5, ECOC 2004, 2004.
Patent Document 1:
Japanese Patent Publication No. 2004-348112
(U.S. Pat. No. 6,798,557: Sep. 28, 2004 “Direct Optical N-State Phase Shift Keying” Appl. No. 443328, Filed: May 22, 2003)
Patent Document 2:
Japanese Patent Publication No. 2002-328347